1. Field of the Invention
The invention relates to a method for screening pulp mixtures with different roughness, which method comprises feeding a pulp mixture to be screened into a screen cylinder from an inlet end of the cylinder, and rotating the screen cylinder around its longitudinal axis during the screening, the pulp being thus screened by means of a screen surface provided with apertures, such that the accepted pulp fraction, or accept, is able to pass the apertures in the screen surface and the rejected fraction, or reject, is conveyed to the opposite or discharge end of the screen cylinder where it is removed from the screen.
2. Field of the Invention
The invention further relates to a screen comprising a screen cylinder arranged to be rotated around its longitudinal axis by a rotator, the inlet end of the screen cylinder comprising a feed pipe for feeding a pulp mixture into the screen cylinder, the circumference of the screen cylinder forming a screen surface, which is provided with apertures of a predetermined size for screening the pulp into an accepted fraction, or accept, that passes said apertures and a rejected fraction, or reject, that does not pass the apertures, the inner circumference of the screen cylinder being provided with conveying means for conveying the reject to the discharge end of the screen cylinder as the cylinder is rotated, the screen comprising a recovery basin for recovering the accept.
3. Description of Related Art
Manufacture of paper includes removal of impurities from pulp and screening of fibre mixtures with different roughness. For example processing of recycled fibre provides pulp mixtures with different roughness consisting primarily of water and fibres, and the mixtures are further processed by screens in order to recover the desired accepted fibre fraction, or accept, and to remove the rejected fraction, or reject. The reject can be conducted for reprocessing or it can be discharged entirely from the process. Pulp mixtures with different roughness have presently their own screens. Screens typically comprise a screen cylinder arranged rotatably around the longitudinal axis, and the pulp to be screened is fed into the cylinder. The circumference of the screen cylinder is provided with apertures forming a screen surface. Depending on the structure the apertures of the screen surface are either holes or slots. During screening the cylinder is rotated and the accepted part of the pulp passes the apertures in the screen surface and is thereafter recovered into a recovery basin and conducted further to subsequent process steps. The material that does not fit through the apertures constitutes the reject, which is supplied inside the cylinder to the discharge end and finally removed therefrom. A problem with the presently used arrangements is that pulp mixtures with different roughness require separate screens. Several screens are naturally expensive to acquire and use and they also require a great deal of space in production plants.